|Estimating biomass and biomass change of tropical forests: a primer. (FAO Forestry Paper - 134) (1997)|
|2. PURPOSE AND SCOPE OF PRIMER ON ESTIMATING BIOMASS|
Decisions by policy-makers regarding the management and use of forest and trees require accurate and precise information on the state and patterns and rates of change of the resource. To attain these needs, reliable estimates on the state and change of forest biomass for all countries over the long term must be made. Biomass estimates for forests of tropical countries, in particular, are needed because globally they are undergoing the greatest rates of change and reliable biomass estimates are few. Their biomass and C content is generally high, which influences their role in the global C cycle. Further, tropical forests have the greatest potential for mitigation of CO2 through conservation and management (Brown et al. 1996).
The main goals of this primer are to (1) present methods that are available for estimating biomass density of tropical forests or tree formations using primarily existing data, (2) give examples of calculations on how the different methodologies are used, (3) describe primary data and measurement requirements to make biomass density estimates, and (4) present biomass density estimates for many tropical countries using the methodologies given. Additional new techniques will be presented that have been shown to be useful for mapping biomass density at regional scales, and that address the change in biomass. It concludes with a discussion on the directions and future studies that are needed to make more reliable estimates of the state and change in forest biomass.
This primer is not intended to be an exhaustive compilation of available information on biomass estimation methodology nor on biomass density estimates for tropical forests. Rather, it is intended to present approaches that have been shown to be useful for application to existing data and to situations where detailed data are lacking, and to emphasize what types of new field data are needed to make improved biomass estimates. Although examples of biomass density estimates for forests in many tropical countries are provided in this primer, they should be used with care when making national biomass inventories because many of the inventories are old (pre-1980s in many cases) and are subject to several of the problems outlined below (see Section 3). Improvements will only occur when estimating biomass density becomes one of the goals of a forest inventory. At this point, collection of the required data for making reliable estimates of biomass will be part of the inventory design. It is expected that the extra data needed should only incur marginal costs. Many inventories already collect most of the required data. Because of the general interest in forest biomass for the reasons given above, many local to national studies on this topic are already being done. The information given in this primer is not meant to replace these efforts, but rather to complement them, and to ensure that basic standards are used (e.g., all biomass density estimates should be based on oven-dry weight).
The methods for biomass estimation presented in this primer are not restricted to just forests but to most formations where trees dominate. This includes closed forests, open forests, woodlands, woody savannas, woodlots, line tree plantings, home gardens, living fences, etc. However, there is a greater choice of available methods for biomass estimation of forest formations than for non-forested lands that contain trees. Forests are defined as land under natural or planted stands of trees with tree crown cover of more than 10 percent and area of more than 0.5 ha, whose primary use is forestry. The trees should be able to reach a minimum height of 5 m at maturity in situ (FAO unpublished report of expert consultation Kotka III). A tree is defined generally as a woody perennial with a single main stem, or in the case of coppice with several stems, having a more or less definite crown and a height greater than breast height (1.3 m).
To address many of the issues raised above related to forest biomass requires that the biomass of all forest components be estimated, including the aboveground and belowground living mass of trees, shrubs, palms, saplings, other understorey components, vines, epiphytes, etc., and dead plant mass such as fine litter and wood. How the mass of these components changes with time and with natural and human disturbances is also important. However, for practical reasons, this primer is restricted to the following component of biomass density: the total amount of live aboveground organic matter present in trees including leaves, twigs, branches, main bole, and bark. The biomass of these components generally accounts for the greatest fraction of total living biomass in a forest and does not pose too many logistical problems in its estimation. Consequently, biomass is defined as the total amount of aboveground living organic matter in trees expressed as oven-dry tons per unit area (tree, hectare, region, or country). It is referred to as biomass density when expressed as mass per unit area, e.g., tons per hectare. The total biomass for a region or country is obtained from the product of biomass density and the corresponding area of forests.
BIOMASS is defined as the total amount of aboveground living organic matter in trees expressed as oven-dry tons per unit area
For most forests or tree formations, biomass density estimates will be based only on the biomass in trees with diameters greater than or equal to 10 cm, which is the usual minimum diameter measured in most inventories of closed forests. However, for forests or trees of smaller stature, such as those in the arid or montane zones, degraded forests, or secondary forests, lower minimum diameters should be chosen.
Woody biomass for many traditional uses does not all originate from forests; significant quantities are obtained from non-forest lands such as small woodlots, wind-breaks, other tree-line formations, home gardens, etc. A methodology for biomass estimation of individual trees, if the primary data are at this level, is presented here as well.